Moving coil diaphragms for electrodynamic listening apparatus



Dec. 25, 1956 e. WURDEL 2,775,653

MOVING COIL DIAPHRAGMS FOR EILECTRO-DYNAMTC LISTENING APPARATUS Filed NOV. 6. 1951 2 Sheets-Sheet 1 IW=E mu 1956 e. WURDEL 2,775,653

MOVING con DIAPHRAGMS FOR ELECTRO-DYNAMIC LISTENING APPARATUS Filed Nov. 6, 1951 2 Sheets-Sheet 2 vlHllll if awn/m? 6 R UUROEL to as centering collar.

is fixed with its end rim by an adhesive.

United States Patent MOVING COIL DIAPHRAGMS FOR ELECTRO- DYNAMIC LISTENING APPARATUS Giinther Wurdel, Berlin-Steglitz, Germany, assignor to Holmberg & (30., Kommanditgesellschatt, Berlin, Germany Application November 6, 1951, Serial N 0. 255,095

Claims priority, application Germany November 16, 1950 6 Claims. (Cl. 179-115) In electrodynamic listening apparatus, such as telephones, microphones, loud speakers and the like, it is required to fix to the diaphragm a thin-walled, sleeveshaped coil body as carrier for the coil winding. This coil body together with the coil winding must be moved within the magnetic field in the annular gap in such a manner that there will be no contact with the poles. In order to obtain as high a performance as possible, it is sought, as always, to make the annular gap as narrow as possible.

In this connection various essential features must be fulfilled. Thus, the coil body must not only be fixed to the diaphragm coaxially and centrally, but care must be taken, that the coil body shall after fixing have a circular form, in other words, must remain circular. Finally, it is essential that the method of fixing shall be sufiiciently reliable.

These requirements have for long been given special attention, without however being fulfilled in every respect in asatisfactory manner. In one group comprising the small diaphragms it is sufiicient to support the diaphragm at a single place, namely near its periphery, in order to ensure adequate parallel guiding during the oscillations. The matter is different, however, in the case of the diaphragms belonging to the group of large surface diaphragms, more particularly the loud speaker diaphragms, where in any case additional parallel guiding is essential. Thisconsists in most cases of a diaphragm fixed at the place of transition between the diaphragm and the coil body and acting as centering means, frequently referred As will be more particularly described below, the invention is applicable most successfully with both diaphragm groups.

It has already been proposed to provide the diaphragm with a concentric annular crease, to which the coil body This, however, does not enable it to be centered in a perfectly satisfactory manner, as there is no accurately defined centering'surface. For overcoming this disadvantage, according to a known proposal in the case of a diaphragm with a central aperture the lower and inner rim has been turned over inwards and the coil body was pushed into the inside of thecylindrical projection produced by the turn-over and the coil body attached with an adhesive to the interior surface of the turn-over. Although this ensured a central arrangement of the coil body, it did not ensure perfectly satisfactory coaxiality or maintenance of the circular shape without the employment of special auxiliary means. Attempts have been made, more particularly in the case of large surface diaphragms, such as loud speaker diaphragms, to fulfill these requirements in a special way with the aid of the centering diaphragm. In such an arrangement the procedure has been, first of all to connect the coil body with the centering diaphragms, care already being taken to ensure an exactly coaxial position of the two parts to one another. Thereupon, the free cylindrical end of the coil body is introduced into the interior of the cylindrical turn-over of the diaphragm, until the lower ice rim of the diaphragm is flushed with the place of attachment of the centering diaphragm with the coil. Finally, the parts are fixed in this position by an adhesive. Only when all this has been done can the whole unit be mounted in the magnet system of the loud speaker.

This constructional form has the disadvantage, that two sources of error as regards coaxiality may creep in, which in the worst case will have an additive effect and thus constitute a specially serious error. One error may be due to the circumstance, that the attachment between the centring diaphragm and the coil is not exactly coaxial and the second error may be due to these two parts not being fixed exactly coaxially with respect to the diaphragm. A further disadvantage resides in this, that the whole units presents difiiiculties in mounting it in the magnet system of the loud speaker, on account of the large surface of the diaphragm, and in centring it, owing to the individual parts not being readily accessible.

According to another known proposal the diaphragm was provided with an annular offset, over which the coil body was pushed from the outside until it bore against the more or less conically extending remaining part of the diaphragm. In this arrangement the central part could be either made closed or provided with an aperture. Whilst with these last-named constructional forms both concentricity and coaxiality could be obtained more reliably than was hitherto possible, such an arrangement could also not satisfy the more exacting requirements as regards coaxiality and concentricity and more particularly the maintenance of the round shape. At best some of these requirements were fulfilled, whilst the others received no due consideration.

Now, the invention has for its object not only to ensure the concentricity and coaxiality, but at the same time the maintenance of the round shape positively with the minimum expenditure of fixing means and with the minimum weight and in addition reliable fixing. For this purpose the annular crease of a kind known per se is given a form which is U-shaped in axial section and deepened to such an extent that two substantially coaxially directed bearing surfaces for the fixing of the coil body result, which leave free between them an annular groove. Into this annular groove the coil body is pushed, until with its end surface it bears right on the bottom of the crease. Although the diaphragm, on account of its domed and conical shape, is relatively rigid in its various parts and not least on account of the U-shaped annular groove, nevertheless in cases of faulty assemblage, where forces directed from the outer rim inwards might occur on one side, there would be a danger of deformation. This deformation might, however, expend itself in the annular groove, through the groove becoming narrower in the places where pressure occurs and wider at other places to a corresponding extent. This however is again not possible, since according to the invention the annular groove is completely filled by the wall thickness of the coil body and the adhesive.

It is on the other hand of special inventive importance, that in assembling the coil body with the diaphragm by pushing the rim of the coil body into the annular groove until the end rim rests on the bottom of the annular groove, not only the concentricity and coaxiality, but also the maintenance of the round shape is positively ensured and in addition a particularly reliable method of fixing. The positive eifect even goes so far that a previously not round coil body is made round through being forced into the annular groove. Of particular advantage in view of the invention is the feature of an unusual correctness of shape taking place after the assemblage, which is obtained in spite of the very small expenditure of material and therefore of weight. On the other hand the positive manner, in which the coil is kept round, is

ensured through its not only being held on one side by a centring surface, but being guided on both sides. A further advantage of the double support consists in the possibility of utilising both surfaces for fixing the coil carrier by means of an adhesive. This ensures a particularly firm connection which is of special importance, as the coil body is exposed during operation to heavy compression and tension stresses. Finally, on account of the U-shaped cross-section of the crease there is the possibility of forming the separate parts of the diaphragm with stronger conicity and limited overall heights, than was the case with the hitherto known diaphragm with a stepped offset, which will be more particularly described hereinafter with reference to the drawings.

Special importance attaches to the material, of which the diaphragm is made. In the case of diaphragms of fibrous material, such as are mainly used for loud speakers, a considerable amount of finishing work is necessary, in order to produce the radius of curvature at the places of transition between diaphragm and coil body, which should be as small as possible for acoustic reasons. The reason for this is to be found in the peculiar mode of manufacture and the structure of the fibre material. In contradistinction thereto it has been found that for the purposes of the invention the use of a synthetic foil, more particularly cellulose Trieste foil on the base of cellulose-acetobutyrate has been found particularly suitable. It is essential for the invention, that the material can be subjected under pressure and heat to every desired deformation. ilore particularly, it can be deformed at the necessary places with very small radii which are not only of advantage for acoustic reasons, but also have increased importance, owing to accurately defined centring possibility.

The accompanying drawings not only illustrate the invention in several constructional examples, but also show by way of comparison examples of the hitherto known methods of fixing a coil body to a diaphragm.

Fig. 1 shows a partial axial section through a diaphragm of knOWn construction, in which the coil body is fixed to a slightly domed crease;

Fig. 2 an axial section through a listening casing with a diaphragm according to the invention having a U-shaped crease for fixing the coil body.

Fig. 3 a partial axial section through the diaphragm according to Fig. 2 to a larger scale at the place of the U-shaped crease.

Fig. 4 an axial section through a diaphragm of known construction with stepped annular offset.

Fig. 5 another known constructional form, namely the mode of fixing a loud speaker diaphragm with the coil body.

Fig. 6 a conical diaphragm similar to that of Fig. 5 with the coil body fixed in accordance with the invention by a U-shaped crease provided at the rim of the diaphragm Fig. 7 the mode of fixing a loud speaker conical diaphragm with a centring diaphragm to the outer sides of the U-shaped crease and Fig. 8 the plan view of a diaphragm with U-shaped crease in the deformed state without the fixed in coil body.

Fig. 1 shows a partial axial section through a diaphragm It of a known kind, which is provided at the place of fixing to the coil body 2 with a crease 3. In this figure may be clearly seen, that the centring possibility is rendered difiicult. since the crease will only enable the coil body to be correctly placed in the central position with the aid of special arrangements or through increased attention when assembling the parts.

According to the constructional example in accordance with the invention, shown Figs. 2 and 3, the diaphragm 6 which is mounted in the enclosing casing 4 with the cover 5 shows a crease '7 of Lfl-shaped crosssection in the axial direction. The depth. of the crease is in this ease up to four times the width. Into the crease the thin-walled moving coil body 8 is pushed so far that it rests firmly on the bottom 9 of the crease. The fixing inside the crease is etfected by means of an adhesive which is at both sides of the coil carrier and ensures a firm connection with the two inner surfaces of the crease.

In the construction of enclosures for listening apparatus, as special value attaches to their having a small fit in depth, that is as small dimensions as possible in the axial direction, only a small amount of space is available in the axial direction for housing the diaphragm, so that the requirement for great rigidity of the diaphragm, which can only be obtained by adequate conicity, is not so easy to fulfill. Now, it proves to be of particularly great advantage, that through the formation of the fixing place of the coil body in the form of a U-shaped crease in accordance with the invention the possibility is presented of giving the separate parts of the diaphragm sufficient conicity, without the overall height of the casing being thereby increased. In this sense it is also particularly advantageous, that according to a special feature of the invention the depth of the U-shaped crease is made as great as possible, namely about 1.2 millimetre, and the width about 0.3 millimetre.

In order to make these proportions particularly clear, the mode of fixing in the hitherto usual manner shall first be considered, as shown in Fig. 4. Although in this known diaphragm the stepped offset 12 allows of the conicity of the inner part 13 being relatively great, whereby the rigidity of the diaphragm will in this part be adequate, it is not possible to make the conicity of the diaphragm parts 14 lying further to the outside and starting from the stepped offset 12 as steep as is desirable with regard to the rigidity to be aimed at. Through the U-shaped formation, however, the possibility is presented of drawing down the outer surface of the diaphragm as well, so that these diaphragm parts 15 (Fig. 2) can be formed with considerably greater conicity. The hitherto p0ssible conicity of these diaphragm parts 14, which in Fig. 4 are shown in full lines, has for comparisons sake been indicated in Fig. 2 as well by dot and dash lines, whilst the conicity of the diaphragm part 15, which can be made considerably greater according to the invention, may be seen clearly from the full lines of Fig. 2. Thus, according to the invention the two limbs of the U-format-ion, which are drawn down deeply in a very favourable manner for fixing, are also very ingeniously made use of for giving the two diaphragm surfaces adjoining these limbs considerable conicity, without the overall height of the diaphragm having thereby to be increased.

In Fig. 5 a conical diaphragm 16 is shown, which belongs to the group of large surface diaphragms and for parallel guiding requires a centring diaphragm 17. This latter diaphragm is stuck to the coil body 8 before being built in, care being taken that the coil body will lie coaxial with respect to the centring diaphragm. Thereupon the coil body is pushed with its upper free rim 18 inside the inwardly turned over cylindrical extension 19 (Fig. 5, left hand side), until the lower edge 20 of the diaphragm comes right against the centring diaphragm 17.

According to another constructional form the coil body 8 was pushed from the outside over the bottom extension 2 1 (Fig. 5 right hand half) of the conical diaphragm, formed by drawing to a cylindrical shape, until it bears right against the place of transition 22 between the extension and the conical surface, and is then fixed by means of an adhesive. But even in these constructional forms difiiculties presented themselves as regards the maintenance of the perfectly round shape and the reliable coaxial fixing between coil body and diaphragm. These difliculties are overcome according to the invention in the manner shown in Fig. 6 through the provision of a U-shaped crease 23 at the inner rim of the diaphragm. In so far as large surface diaphragm are concerned a centring diaphragm 24 may be provided, as shown to the left in Fig. 6, or, as in the case of small diaphragms, this centring diaphragm may be omitted as in the right hand part of Fig. 6. In this construction no difliculties are presented as regards the coaxial fixing, as care need only be taken, that the coil body shall rest firmly at the bottom of the annular groove.

How extremely rigid the diaphragm will be after the insertion of the coil body will be seen from the following consideration. -Should compressive forces be exerted on the diaphragm at two opposite places '(a, Fig. 8) in the direction of the surface from the outside inwardly, as might occur, should the part be inserted incorrectly, then, should there he no coil body in the annular groove, the annular groove would the the place of least. resistance in the otherwise relatively rigid diaphragm. The annular groove would contract at the places b and expand at the places lying at right angles to them. This is, however, no longer possible after the insertion of the coil body, as the annular groove is completely [filled by the coil body and the adhesive. From what has been stated it is quite clear, how greatly the annular groove as a specially formed place of connection with the coil body contributes to the stiffening of the whole structure consisting of the diaphragm and the coil body.

The new diaphragm may also be used with good effect in accordance with the invention as centring means for a large surface conical diaphragm (Fig. 7), which is fixed by an adhesive at the lower rim with an inwardly bent over (Fig. 7 left hand portion) or an outwardly drawn (Fig. 7 right hand portion) cylindrical extension to the outer or inner limb of the U-shaped crease of the centring diaphragm, forming the annular groove. As in this case the centring diaphragm only has to perform the tunction of an additional parallel guide, it is not built up out of part surfaces having the :stiflening action of a cone, but is constructed in .an elastically easily deformable manner in an annular corrugated or annular sineshaped form. In the centr-ing diaphragm a plurality of small holes is provided or it is perforated in a sieve-like manner, the apparatuses being so small that foreign bodies or gr-it will be effectively kept away from the annular gap.

This construction great-1y facilitates the hitherto extremely awkward assembling of loud speakers. Thus, the centring diaphragm can first he mounted in the magnet system and only then need the large surface diaphragm be connected with the inner and outer side of the crease.

I claim:

1. In a sound reproducing apparatus, in combination, a diaphragm having an annular channel-shaped projection, including two annular concentric parallel side walls extending substantially normal to the plane of said diaphragm, and a bottom wall, said channel-shaped projection having a U-shaped cross section so that said side Walls and said bottom wall define an annular channel; a coil support having an annular end portion projecting into said channel and engaging the inner face of said bottom wall, said annular end portion having coaxial annular outer and inner surfaces extending parallel to said side walls within said annular channel and engaging said side walls to hold said coil support in a position normal to the plane of said diaphragm and so as to reinforce said channel-shaped projection, said channel carrying said coil support and positioning the same accurately with respect to the diaphragm, and said channel acting as a gauge to guarantee that a coil support of proper size and wall thickness is connected to the diaphragm; and a coil arranged on said coil support outside of said channel.

2. In a sound reproducing apparatus, in combination,

a diaphragm having a central axis, said diaphragm having a circular channel-shaped projection projecting from said diaphragm and being conccutric with said axis of the same, said channel-shaped projection including two cylindrical concentric parallel side walls having a common axis coinciding with saidaxis of said diaphragm and a bottom wall so as to define a circular channel in said channel-shaped projection; a cylindrical coil support having an axis coinciding with said axis of said diaphragm and having a cylindrical wall end portion projecting into said channel, said wall end portion engaging the inner face of said bottom wall and having cylindrical coaxial outer and inner surfaces extending parallel to said cylindrical side Walls Within said circular channel and engaging said side walls to hold said coil support in a position coaxial with said diaphragm and so as to reinforce said channel-shaped projection, said channel carrying said coil support and positioning the same accurately with respect to the diaphragm, and said channel acting as a gauge to guarantee that a coil support of proper size and wall thickness is connected to the diaphragm; and a coil arranged on said coil support outside of said channel.

3. A device as claimed in claim 2 wherein said diaphragm is made of thermoplastic synthetic foil.

4. A device as claimed in claim 2 wherein said diaphragm is made of a cellulose Trieste foil on the base of cellulose acetooutyrate.

5. in a sound reproducing apparatus, in combination, a diaphragm having a central axis and being formed with a circular opening in the center thereof, said diaphragm having a circular channel-shaped projection projecting from said diaphragm and being concentric with said axis of the same, said channehshaped projection bounding said circular opening, said channel-shaped projection including two cylindrical concentric parallel side walls having a common axis coinciding with said axis of said diaphragm and a bottom wall extending substantially normal to said side walls so as to define a circular channel in said channel-shaped projection; a cylindrical coil support having an axis coinciding with said axis of said diaphragm and having a cylindrical wall end portion having a circular edge and projecting into said channel, said wall end portion engaging said circular edge thereof the inner face of said bottom Wall and having cylindrical coaxial outer and inner surfaces extending parallel to said cylindrical side walls within said circular channel and engaging said side Walls to hold said coil support in a position coaxial with said diaphragm and so as to reinforce said channel-shaped projection, said channel carrying said coil support and positioning the same accurately with respect to the diaphragm, and said channel acting as a gauge to guarantee that a coil support of proper size and wall thickness is connected to the diaphragm; and a coil arranged on said coil support outside of said channel.

6. A device as claimed in claim 2 and wherein said channel formed in said channel-shaped projection of said diaphragm has a depth of approximately 1.2 millimeters and wherein said concentric parallel side walls thereof are spaced from each other a distance of approximately 0.3 millimeter.

References Cited in the file of this patent UNITED STATES PATENTS 2,207,240 Cornwell July 9, 1940 2,231,479 Perry Feb. 11, 1941 2,351,904 Ballinger June 20, 1944 2,371,951 Cook Mar. 20, 1945 2,392,143 Graham Jan. 1, 1946 2,534,040 Lindley Dec. 12, 1950 2,640,556 Brennan June 2, 1953 

